From 2005 to 2008, one program was held in Cambridge, Massachusetts, and one was held in Mountain View, California. In January 2009, Y Combinator announced that the Cambridge program would be closed and all future programs would take place in Silicon Valley.[4]

In 2009, Sequoia Capital led the $2 million investment round into an entity of Y Combinator which would allow the company to invest in approximately 60 companies a year as opposed to their previous 40 companies a year.[5] The following year, Sequoia led a $8.25 million funding round for Y Combinator to further increase the number of startups the company could fund.[6]

Then, in 2011, Yuri Milner and SV Angel offered every Y Combinator company a $150,000 convertible note investment.[7] The amount put into each company was changed to $80,000 when Start Fund was renewed.[8]

In September 2013, Paul Graham announced Y Combinator would fund nonprofit organizations accepted into its program after having tested the concept with Watsi (while continuing to fund mostly for-profit startups).[9]

In 2014, founder Paul Graham announced he was stepping down and that Sam Altman would take over as President of Y Combinator.[10][11] That same year, Altman announced "The New Deal" for YC startups, which offers $120,000 for 7% equity.[12][13]

Late in 2014, Sam Altman announced a partnership with Transcriptic to provide increased support for Y Combinator's growing community of biotech companies.[14] Then in 2015, he announced a partnership with Bolt and increased support for hardware companies.[15]

In 2015, Y Combinator announced a closure of its $700 million growth fund led by Ali Rowghani.[11]

In September 2016, Y Combinator announced shuffling the deck at the Mountain View startup accelerator again, with Altman announcing that he will now be president of YC Group, which includes Y Combinator, the YC Continuity fund that was launched last October and the YC Research "moon shot" program.

Ali Rowghani, Twitter's former chief financial officer and chief operating officer who was put in charge of the YC Continuity Fund when it started, is now CEO of YC Continuity. Michael Seibel, who co-founded Justin.tv, is the new CEO of YC Core, the program that Paul Buchheit has run since earlier this year.[18]

In its main program, Y Combinator interviews and selects two or more batches of companies per year. The companies receive seed money, advice, and connections in exchange for 7% equity.[19] The program includes "office hours", where startup founders meet individually and in groups with Y Combinator partners for advice. Founders also participate in weekly dinners where guests from the Silicon Valley ecosystem (successful entrepreneurs, venture capitalists, etc.) speak to the founders.

Y Combinator’s motto is "Make Something People Want."[20] The program aims to focus the founders on further developing their product, team and market, refining their business model, achieving product/market fit, and scaling the startup into a high growth business, etc. The program culminates at Demo Day where startups present their business to a selected audience of investors.[21]

Non-profit organizations can also participate in the main YC program.[24]

In 2015, YC introduced additional programs:

In July 2015, Y Combinator introduced the YC Fellowship Program aimed at companies at an earlier stage than the main program.[25]

In October 2015, Y Combinator introduced the YC Continuity Fund. The fund allows Y Combinator to make pro rata investments in their alumni companies with valuations under $300 million. Y Combinator will also consider leading or participating in later stage growth financing rounds for YC companies.[26]

In November 2014, Ali Rowghani joined YC as a part-time partner focusing on helping YC alumni scale their companies.[37][38] He is listed on the YC website as the managing partner of YC Continuity.[39]

In August 2018, Y Combinator announced its hiring of a former executive of Microsoft and Baidu Qi Lu to be the Head of YC Research and to develop YC China.[40]

Y Combinator has been blamed for its encouragement of the ageism culture in Silicon Valley. Paul Graham said in 2005 that people over 38 lacked the energy to launch startups.[42] It was also at a Y Combinator event, the 2007 Startup School, that Mark Zuckerberg said, "Young people are just smarter".[43] The organization has been similarly criticized for reflecting the culture of sexism in the tech industry.[44]

The YC Fellowship Program was announced in July 2015, with the goal of funding companies at the idea or prototype stage.[25] The first batch of YC Fellowship included 32 companies that received an equity-free grant instead of an investment.[23]

In January 2016 Y Combinator announced version 2 of the program, with participating companies receiving $20k investment for a 1.5% equity stake. The equity stake is structured as a convertible security that only converts into shares if a company has an IPO, or a funding event or acquisition that values the company at $100m or more.[45] The YC Fellowship was short lived, however, as in September 2016 then CEO Sam Altman announced that the fellowship will be discontinued. In 2017, Y Combinator announced Startup School, an online course that released public videos and also coached individual startups on a larger scale to replace the fellowship. 1584 startups graduated the program in its first year.[46] In 2018, Y Combinator announced a new batch of startup school. After a software glitch, all 15,000 startups that applied to the program were accepted, only to learn a few hours later that they had been rejected.[47]

Nonprofit research lab YC Research was announced in October 2015. Researchers are paid as full-time employees and can receive equity in Y Combinator.[27][48][49]OpenAI was the first project undertaken by YC Research, and in January 2016 a second study on basic income was also announced.[50] Another project is research on new cities.[51]

The Human Advancement Research Community (HARC) project was set up with the "... mission to ensure human wisdom exceeds human power ...".[52][53] The project was inspired by a conversation between Sam Altman and Alan Kay.[54] Its projects include modelling, visualizing and teaching software, as well as programming languages. Its members include Alan Kay and Bret Victor. Other people who have worked for HARC include Vi Hart. Patrick Scaglia was chair of HARC and was listed as an advisor in 2017.[55][56]

In 2017 Forbes ranked YC one of two "Platinum Plus Tier U.S. Accelerators".[57]Fast Company has called YC "the world's most powerful start-up incubator".[58]Fortune has called Y Combinator "a spawning ground for emerging tech giants".[59]